Dow company reported [Me2Si(Me4C5)NtBu]TiCl2 (Constrained-Geometry Catalyst, CGC) in the early 1990s (U.S. Pat. No. 5,064,802). The advantages of CGC in comparison to the previously known metallocene catalysts in the copolymerization reaction of ethylene with alpha-olefin may be summarized into the following two: (1) it shows high activity even at a high polymerization temperature to result in a polymer having a high molecular weight, and (2) it gives excellent copolymerization of alpha-olefins having high steric hindrance such as 1-hexene and 1-octene. Besides the above, as several characteristics of CGC were gradually known, many derivatives thereof have been synthesized and actively used as a polymerization catalyst in academic and industrial fields.
As one approach thereof, it was tried to synthesize a metal compound to which other various bridges instead of a silicon bridge and nitrogen substituent were introduced and to use it for polymerization. Representative metal compounds known hitherto may be listed as follows (Chem. Rev. 2003, 103, 283).

In the compounds listed above, phosphorus (1), ethylene or propylene (2), methylidene (3) and methylene (4) bridges are respectively introduced instead of the silicon bridge of the CGC structure. However, they do not show excellent results in the aspects of polymerization activity, copolymerization performance, etc. in comparison to CGC when they are applied to the ethylene polymerization or copolymerization of ethylene with alpha olefin.
As another approach, many compounds comprising an oxido ligand instead of the amido ligand of CGC have been synthesized, and some polymerizations using them have been tried. Examples thereof may be summarized as follows:

Compound (5) was reported by T. J. Marks, et al., and is characterized in that the Cp (cyclopentadiene) derivative and the oxido ligand are cross-linked via the ortho-phenylene group (Organometallics 1997, 16, 5958). A compound having the same cross-linking and polymerization using the same have been reported by Mu et al. (Organometallics 2004, 23, 540). Furthermore, a product produced by cross-linking an indenyl ligand and an oxido ligand via an ortho-phenylene group was reported by Rothwell, et al. (Chem. Commun. 2003, 1034). Compound (6) was reported by Whitby, et al. and is characterized by bridging the cyclopentadienyl ligand and the oxido ligand via three carbon atoms (Organometallics 1999, 18, 348). These catalysts were reported to show an activity for the syndiotactic polystyrene polymerization. A similar compound has also been reported by Hessen, et al. (Organometallics 1998, 17, 1652). Compound (7) was reported by Rau et al. and is characterized by showing the activity in the copolymerization of ethylene with ethylene/1-hexene at high temperature and pressure (210° C., 150 MPa) (J. Organomet. Chem. 2000, 608, 71). In addition, subsequently, catalysts having similar structures such as Compound (8) were provided, and polymerization using the same at high temperature and pressure was filed by Sumitomo company (U.S. Pat. No. 6,548,686).
However, among all the above trials, only a few catalysts are actually applied in the industrial companies. The polymers produced using the commercially available catalysts also have the disadvantage of poor heat resistance despite the high copolymer content. They usually have the melting point around 50° C. to 70° C. and thus show the demerit of not being suitable for the use requiring the heat resistance of 100° C. or more.